Laser cooling of nuclear spin 0 alkali $^{78}$Rb

The textbook example for sub-Doppler cooling is a J=1/2 I=0 alkali atom in lin $\perp$ lin molasses. In the $\sigma^+$ $\sigma^-$ configuration of a standard MOT, the main sub-Doppler cooling mechanism relies on changing alignment ($M_F^2$ population) with the summed linear polarization orientation, but there is no such variation in AC Stark shift for F=1/2. We have nevertheless looked for signs of sub-Doppler cooling by trapping I=0 $^{78}$Rb in a standard MOT and measuring the cloud size as a function of laser detuning and intensity. The $^{78}$Rb cloud size does not change significantly with lowered intensity, and expands slightly with detuning, consistent with minimal to no sub-Doppler cooling. Our geometry does show the well-known substantially smaller cloud size with detuning and intensity for I=3/2 $^{87}$Rb. Maintaining an I=0 alkali cloud size with lowered intensity will help our planned $\beta$-$\nu$ correlation experiments in $^{38m}$K decay by suppressing possible production of photoassisted dimers.